fmt: rename errno and error to err for doc consistency
fmt: rename errno and error to err for doc consistency

The public godoc looked confused. I imagine these were
written before current conventions were established.

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/4662060

tav/go,tav/go,tav/go,tav/go,tav/go,tav/go,tav/go